As the wireless local area network (WLAN) becomes ubiquitous, the users of portable devices can easily find access points to surf the internet and obtain the information without the restriction imposed by the conventional wiring cables. However, the electromagnetic wave (EM wave) emitted from the wireless communication is potentially harmful to the health, and can also interfere with other electronic instruments. Therefore, its use is usually restricted in the environment where sensitive electronic devices are widely used, such as in a hospital or an aircraft. In addition, because wireless electronic networks are susceptible to eavesdropping or interception, they are unsuitable for applications such as military, government or financial institutes where data security is highly regarded. On the other hand, wireless optical communication provides the alternative to avoid the disadvantages of using electronic wireless communication, including health, security and interference of EM waves. Furthermore, optical wireless communication is not regulated by the government in terms of frequency spectrum management; hence, optical wireless communication provides great potential for a wider range of applications.
Conventional optical wireless communication systems use optical filters, such as low-pass or band-pass filters, to filter out light of different wavelength. Then, an electronic high-pass filter is used to filter out the low frequency harmonic interference. FIG. 1 shows the harmonic interference from various artificial light sources. Although the use of high-pass filter can achieve the suppression of interferences, the cut-off frequency of the electronic high-pass filter must be raised in order to filter out the interferences entirely. This, however, may lead to the distortion of the signals, and generating the additional inter-symbol interference (ISI) effect. Therefore, in conventional technologies, the cut-off frequency of the high-pass filter must be designed to optimize the balance between the increases of ISI and the interference suppression. But the conventional methods can only provide 0.2 dB of improvement in filtering the interference of fluorescent light. The unfiltered fluorescent interference still creates a loss of 17 dB or more, and is unable to reject the interference entirely. The currently available solutions for this include sub-carrier modulation, line coding techniques, spread spectrum modulation, differential detection and adaptive threshold detection techniques.
Conventional sub-carrier modulation techniques are used in optical wireless communication. The signals are modulated to higher frequency range to avoid the overlapping of signal frequency spectrum with interfering optical spectrum. Then, an electronic high-pass filter can be used to effectively filter the interfering source. However, these techniques require additional modulation and demodulation mechanisms, and the system complexity is increased.
Line coding techniques are similar to the sub-carrier modulation techniques, except that these techniques add line coding to modulate the signal to the higher frequency to avoid the interference. For example, U.S. Pat. No. 5,600,471/1997, by JVC, disclosed an optical wireless data transmitting and receiving apparatus using a Manchester line coding to reject interference, and U.S. Pat. No. 5,706,115/1996 disclosed a method of using differential mode inversion (DMI) line coding for interference rejection in an optical wireless communication system. Although these techniques are simple, the disadvantages including the bandwidth requirement twice of that of conventional non return to zero (NRZ), the increasing received optical noise, transmission distortion, and higher demands on the optical components. FIG. 2 shows a waveform view of various line coding techniques, including NRZ, Manchester, DMI and delay modulation.
The spread spectrum modulation techniques use spread spectrum to disperse the base frequencies in order to reduce the impact on the signal. These techniques greatly increase the system complexity, transmission bandwidth requirement and impose higher demands on optical components. Most of the researches in this area are academic, and few are applied to industrial use.
The different detection techniques, originally proposed by U.C. Berkeley, use two optical receivers, with one of them passing through an optical filter and the other passing through a tunable attenuator. The interference-free signal can be extracted by the difference between two signals. The disadvantages of these techniques include the complexity of front-end circuitry and the uncertainty of interference improvement dependent on interference characteristics.
The adaptive threshold detection techniques change the detection threshold at the receiving end according to the condition of the received signal. These techniques can improve the change of the signal bias, but are less effective for optical interference and these techniques increase the front-end circuitry complexity.
In summary of the aforementioned analysis, most optical wireless communication interference-rejection techniques focus on modulation and analog front-end processing. The line coding techniques, in comparison, exhibit a better performance in interference-rejection while only slightly increase system complexity. Therefore, if the bandwidth requirement problem can be solved, the line coding techniques can be more effectively applied to optical wireless communication systems.
As the general health concern, wireless optical communication has a restrictive standard (ILC-825-1) on transmission power. This, combined with the high propagation attenuation (20-40 dB) of the air, greatly limits the power of the signals received at the receiving end. In addition, the indoor light sources, such as sun rays, fluorescent light, lamps, infra-red remote controllers, all contribute to the optical interference in the optical wireless communication. As the interference severely degrades the S/N ratio at the receiving end, it is imperative to solve the problem of optical interference.